1. Field of the Invention
The present invention generally relates to an apparatus and a process for removing ceramic materials from, and cleaning the surfaces of, articles and specifically relates to improved apparatus and processes for removing ceramic material and cleaning loose and tightly bound contamination from the surfaces of airfoil components on a production basis.
2. Description of the Prior Art
U.S. Pat. No. 5,685,917 to Sangeeta entitled xe2x80x9cMethod for Cleaning Cracks and Surfaces of Airfoilsxe2x80x9d, U.S. Pat. No. 5,643,474 to Sangeeta entitled xe2x80x9cThermal Barrier Coating Removal on Flat and Contoured Surfacesxe2x80x9d and U.S. Pat. No. 5,779,809 to Sangeeta entitled xe2x80x9cMethod of Dissolving or Leaching Ceramic Cores in Airfoilsxe2x80x9d explain the use of an organic caustic mixture under pressure for the cleaning and removal of ceramic materials such as ceramic cores used in the production of casting gas turbine hardware and thermal barrier coatings used to improve the temperature capabilities of gas turbine hardware. The processes outlined have several problems that must be overcome to practice the technology in production environments with higher throughput. Basically, the patents describe methods of attacking the ceramic materials by exposing them under elevated temperature and pressure to organic caustic solutions comprised of a volatile organic compound, a caustic compound and water.
The reagents involved are highly alkaline and flammable, a combination that renders them particularly difficult to handle. The pressures and temperatures set forth in these patents are high, being elevated well above ambient, thereby causing the entire process to be extended in duration. While this is acceptable for laboratory settings or in small scale runs, it is undesirable in production settings. These prior art processes comprise loading a pressure vessel such as an autoclave, with soiled, coated turbine hardware and adding the caustic reagents. The loaded pressure vessel is brought to the appropriate elevated temperature and pressure, thereby subjecting the coated parts to the caustic reagents which act on the hardware to remove the coating. The pressure vessel is then cooled and depressurized and the stripped hardware is removed from the vessel. The hardware is then removed from the vessel and residual reagents are removed from the hardware. However, these prior art processes are not readily adaptable for the high volumes usually encountered in production situations. The prior art processes do not address the problems of adapting such autoclave equipment, typically designed for batch processing, for continuous production processing. Nor do the prior art processes address the problems encountered in reusing these contaminated and dangerous chemicals.
What is needed are equipment and methods capable of removing ceramic materials such as coatings from coated hardware as the first step in a process for refurbishment and restoration of turbine hardware in an efficient and safe manner, while eliminating contamination from the reagent to allow reuse.
The present invention is directed to a combination of equipment that provides apparatus and a method for conveniently removing ceramic coatings from, and cleaning the surfaces of articles using a caustic solution such as an alkaline hydroxide. This invention provides the ability to process a large quantity of articles in a short period of time while providing the capability to reuse the caustic chemicals for multiple cycles of article processing.
The apparatus of the present invention includes means for storing the caustic solution until it is ready for application to the articles. When ready for use, the caustic solution is preheated to a first preselected temperature by a means for preheating. The means for preheating may be a separate chamber or may be a device such as a heating coil which elevates the temperature of the solution as it exits the means for storing. The caustic solution is then pressurized to a first pressure by a means for pressurizing. The pressurization may be accomplished in the same device as the preheating. The pressurization may be performed in conjunction with the preheating. The caustic solution, preheated to a first temperature and pressure is now introduced into a pressure vessel by a suitable means for introducing and transferring the caustic solution. As will become clear, the processes of the present invention result in the pressure vessel being at an elevated temperature above ambient. The pressure vessel, prior to introduction of the heated, pressurized caustic solution, is loaded with the articles which are to be processed. These articles require processing to remove or strip ceramic coating as a first step to reprocessing. As the hardware has typically been utilized in a gas turbine, not only must the ceramic coating be removed, but also undesirable materials, such as loose contamination including soot and other by-products of fuel combustion, and tightly adherent oxides resulting from the high temperatures of combustion, must be removed.
The pressure vessel has an internal volume that is substantially larger than any of the articles which are to be stripped and also has the capacity to receive a substantial amount of caustic solution. The pressure vessel also has the ability to achieve pressures and temperatures well in excess of ambient. After a plurality of articles are loaded into the pressure vessel and the caustic solution at a first elevated temperature and pressure have been introduced into the hot pressure vessel, the vessel and its contents may require some minor heating to equalize the temperature of the vessel and its contents at the first elevated temperature, as some heat may be lost during the loading and unloading processes. In an optional embodiment, the pressure vessel and its contents may be heated to a preselected second elevated temperature above the first preselected temperature by a second heating means. The pressure vessel also may be raised to a preselected second elevated pressure above the first preselected pressure.
The pressure vessel and its contents are then held at temperature and pressure for a sufficient time to permit the caustic solution to interact with the surface of the articles so as to either remove the materials overlying the substrate or to weaken such materials substantially so that they can be removed with little additional effort, while not otherwise affecting the article substrate. After sufficient time at pressure and temperature has passed to accomplish the desired result of stripping or substantial weakening of materials on the substrate of the article, the caustic solution is removed from the pressure vessel by a suitable means for removing the solution. Of course, the removal of the solution may cause a drop of pressure in the vessel. The caustic solution is then cooled by a means for cooling after its removal from the pressure vessel. After cooling to a suitable temperature, the solution can then be safely transferred to the means for storing the solution, until the next cycle of operation is ready to commence.
The articles within the pressure vessel may now be removed for further processing, while the pressure vessel remains hot. However it will be necessary to rinse the caustic solution from the articles after stripping. This is accomplished by use of a second vessel and introduction of a suitable reagent, which can include water. The reagent will also serve to sufficiently cool the articles so that their removal from the second vessel can be expedited without substantially lowering the autoclave temperature.
Improvements in manufacturing technology and materials are the keys to increased performance and reduced costs for many articles. Here, continuing and often interrelated improvements in processes and materials results in the ability to remove materials overlying a substrate, which substrates typically are expensive alloys, without harming the underlying substrate. This allows for improved ability to refurbish articles without adversely affecting the engineering properties of the articles.
An advantage of the present invention, therefore, is an improved ability to remove ceramic coatings from expensive articles without adversely affecting the underlying articles. The articles can thus be refurbished without any impact on the engineering properties of the articles. This in turn increases the useful life of the articles and avoids the need to prematurely replace the articles with expensive new articles, thereby conserving scarce resources.
Another advantage of the present invention is the ability to reuse and recycle caustic solutions. By reuse, not only is the cost of replacing the caustic solutions avoided, but the disposal of the caustic solution is avoided, thereby contributing to an improved environment.
Still another advantage of the present invention is that highly alkaline and flammable reagents that are difficult to handle can now be used in the processing of articles in a production environment at elevated temperatures and pressures safely and with minimal human contact.
Still another advantage of the present invention is the ability to reduce the cycle time for stripping or cleaning. The present invention maintains the pressure vessel at a substantially elevated temperature as parts are cycled through it, thereby eliminating cool down cycles. This eliminates the substantial heat up time for the pressure vessel which typically has a large thermal mass. While shortening cycle time, it also reduces energy consumption, both of which translate into cost savings.
Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings which illustrate, by way of example, the principles of the invention.